Production of silicates



Feb.

11, 1958 l. MOCKRIN 2,823,099

PRODUCTION OF SILICATES Filed May 28, 1954 NOTE COEFFICIENT REFERS TOMOLS OF CATALYST PER MOL OF SILICA.

M8 00, sa-o1 Na- SW 0 I l v T l M E HOURS INVENTQR. 15411-12 fi 1 UnitedStates Patent PRODUCTION OF SILICATES Isadore Mockrin, Philadelphia,Pa., assiguor to Pennsalt Chemicals Corporation, a corporation ofPennsylvania Application May 28, 1954, Serial No. 433,033 Claims. (Cl.23-110) The present invention relates to the manufacture of silicatesand more particularly to improvements in the process of manufacturingsilicates wherein the silicate is prepared by a substantially dry statereaction between an alkali metal carbonate and, silica.

It has heretofore been proposed to react an alkali metal carbonate, suchas, sodium carbonate with silica in a substantially dry state to formsodium silicate. The process is. particularly desirable from thestandpoint of economy since sodiumcarbonate is substantially lessexpensive thansodium hydroxide as a source material for the Na- O. Thereactions have the objectionable feature, however, that temperatures inexcess of 850 C. have been necessary in order to obtain asuflicientlyrapid reaction. to make the process commercially feasible.

At temperatures much in excess of 850 C. a melt occurs, particularlyduring the early stages of the reaction, which is corrosive and tends toattack the equipment in which the reaction is being carried out.

In order to avoid any substantial occurrence of melt while carrying outthe reaction between the sodium carbonate and the silica, it has beenproposed to first heat a mixture of the reactants to a temperature below850 C. for a time sufiicient to volatilize substantial amounts of CO andthereafter raise the temperature (still maintaining it as a temperaturebelow which any melt occurs) to increase the rate of reaction. In thismanner the reaction can be carried out at sufficiently high temperaturesto obtain a reasonable reaction rate while avoiding the occurrence ofany substantial amount of melt. This process, however, has the inherentdifficulty of requiring careful control in order to be sure that thetemperature is not elevated too soon. Also, the high temperaturesrequired add to the expense of obtaining the final product and aregenerally objectionable.

In my co-pending application Serial No. 433,031, filed of even dateherewith, i. e., May 28, 1954, a process is described whereinsubstantially dry state reactions between alkali metal carbonates andsilica are catalyzed by the inclusion of small amounts of inorganichalogen compounds. The present invention is drawn to the furtherdiscovery that these reactions are also catalyzed by inorganictetraborates.

By the addition of small amounts of inorganic tetraborates to thereaction mixture, the reaction can be substantially accelerated attemperatures well below that at which any melt occurs. The inorganictetraborates have been found to act as catalysts even when present inamounts as low as 0.03 mol inorganic tetraborate per mol of SiO present.

The surprising catalytic efiect obtained by the addition of the abovementioned inorganic tetraborates is well illustrated by the accompanyingdrawing which contains curves showing the reaction rate at 700 C. forthe reactants, sodium carbonate and silica in the presence of aninorganic tetraborate catalyst and in the absence of any catalyst.

The curves, which are the results of actual experiments,

2,823,099 Patented Feb-11, 1958 "ice are given for the purpose ofillustration, and the invention is not limited to the specific inorganictetraborate shown. I believe that all inorganic tetraborates have thiscatalytic efiect. Thus, any of the. compounds CaB O Li B O and K B O maybe added as a' catalyst to catalyze the reraction in place of thespecific inorganic tetraborate shown in the drawing. The preferredinorganic tetraborates are those of sodium and potassium since noinsoluble products would be brought into the reaction prod not throughtheir use.

In practicing the present invention, temperatures as low as 600 C. canbe used. However, it is generally preferred to carry out the reactionbetween the alkali metal carbonate and the. SiO, source material at atemperature within the range of 650 C. to just below the temperature atwhich any substantial melt may occur. This upper limit will in generalbe approximately 850 C. at atmospheric pressure during the early stagesof the reaction for sodium carbonate. For the other alkali metalcarbonates it will vary with their melting temperatures. However,somewhat higher temperatures may be used if desired as long: as nosubstantial amount of melt occurs, since the presence of an inorganictetraborate accelerates the reaction even at thehigher temperatures.Some mixtures of silica and alkali metal silicates have somewhat lowermelting temperatures than the alkali metal carbonates used. In suchinstances, the'temperature should be maintained below the melttemperature of the product.

It is generally preferred to use a finely-divided silica, the preferredparticle size generally being minus 325 mesh. However, the process canbe practiced successfully with silica in particle sizes as high as --65mesh.

The rate of the reaction can be varied markedly by varying the ratio ofalkali metal carbonate to silicon di oxide. In practicing the presentinvention, the reactants are generally present in a silica to alkalimetal carbonate to mol ratio of 1:2 to 3:1. Where there is no objectionto having some unreacted silica in the final product, the preferredratio of silica to alkali metal carbonate in the reaction mixture isabout 1.5:1 to 3:1.

The rate of reaction can be increased still further by substituting analkali metal hydroxide for part of the alkali metal carbonate. Asurprising result obtained by substituting an alkali metal hydroxide forpart of the alkali metal carbonate is that the resulting products giveclearer solutions, when dissolved in water, than those prepared fromreactants containing no alkali metal hydroxides.

Where the alkali metal carbonate is replaced in part by the alkali metalhydroxide, the alkali metal hydroxide is preferably used in amounts of0.1 to 0.7 mol alkali metal hydroxide per mol alkali metal carbonate.

In order to better illustrate the practice of the present invention, thefollowing example is given:

Sodium silicate was prepared by first grinding together for about twominutes 6.01 parts by weight sodium carbonate, 3.41 parts by weightsilica of minus 325 mesh size and 0.57 part by weight of sodiumtetraborate. The mixture corresponded to the following formulation:

The ground reactants were heated for about five hours at 700 C. afterwhich time the reaction was found to be 66% complete.

As previously stated, any of the inorganic tetraborates can be used ascatalysts and any of the alkali metal carbonates may be employedtogether with a suitable source of SiO The conditions of the reactionwould be substantially the same as those of the above example.

Having thus described my invention, I claim:

1. In the preparation of alkali metal silicates, the process comprisingheating, in a substantially dry state, a

mixture containing an alkali metal carbonate, silicon dioxide and aninorganic tetraborate to a temperature of from 600 C. to a temperaturebelow that of which any appreciable melt occurs, the mol ratio ofsilicon dioxide to alkali metal carbonate in said mixture being Withinthe range of from 1:2 to 3:1.

2. The process of claim 1 wherein the inorganic tetraborate is atetraborate of a metal of the group consisting of sodium and potassium.

3. In the preparation of alkali metal silicates the process comprisingheating, in a substantially dry state, a mixture containing an alkalimetal carbonate, silicon dioxide and an inorganic tetraborate, in whichsaid alkali metal carbonate is selected from the group consisting ofsodium carbonate, potassium carbonate, rubidium carbonate and caesiumcarbonate, and in which the mol ratio of silicon dioxide to alkali metalcarbonate is within the range of 1:2 to 3:1, said heating being at atemperature Within the range of 650 C. to a temperature just below thatat which any substantial melt occurs.

4. The process of claim 1 wherein said inorganic tetraborate is presentin amounts of at least .03 mol inorganic tetraborate per mol silicondioxide.-

5. The process of claim 1 wherein the silicon dioxide to alkali metalcarbonate mol ratio is Within the range of 1.5:1 to 3:1.

6. In the preparation of alkali metal silicates the proc ess comprisingheating, in a substantially dry state, to a temperature of 600 C. tojust below where any appreciable melt occurs, a mixture consistingessentially of an alkali metal carbonate, an alkali metal hydroxide,silicon dioxide, and a small amount of an inorganic tetraborate, inwhich mixture the mol ratio of silicon dioxide to the 0 sum of thealkali metal oxide content of the alkali metal carbonate and alkalimetal hydroxide is 1:2 to 3:1 and in which said alkali metal hydroxideis present in amounts of 0.1 to 0.7 mol alkali metal hydroxide per molalkali metal carbonate.

7. The process of claim 6 wherein said inorganic tetraborate is atetraborate of a metal of the group consisting of sodium and potassium.

8. The process of making sodium metasilicate comprising reacting in asubstantially dry state sodium carbonate with silicon dioxide in thepresence'of an inorganic tetraborate at a temperature within the rangeof 600 C. to a temperature just below that at which any appreciable meltoccurs.

9. The process of claim 8 wherein theinorganic tetraborate is atetraborate of a metal of the group consisting of sodium and potassium.

10. The process of claim 8 wherein the silicon dioxide to sodiumcarbonate mol ratio is within the range of 1:2 to 3:1 and the reactionis carried out at a temperature of 650 to 850 C.

References Cited in the file of this patent UNITED STATES PATENTS Re.21,703 Burkhart Feb. 4, 1941 2,219,646 Beecher Oct. 29, 1940 2,239,880Curll Apr. 29, 1941 2,357,723 Beecher et al. Sept. 5, 1944 2,374,035Nutting Apr. 17, 1945 OTHER REFERENCES Howarth et al.: J. of Soc. ofGlass Technology, vol. 17, Transactions, 1933, pages 25-49.

1. IN THE PREPARATION OF ALKALI METAL SILICATES, THE PROCESS COMPRISINGHEATING IN A SUBSTANTIALLY DRY STATE, A MIXTURE CONTAINING AN ALKALIMETAL CARBONATE, SILICON DIOXIDE AND AN INORGANIC TETRABORATE TO ATEMPERATURE OF FROM 600*C. TO A TEMPERATURE BELOW THAT OF WHICH ANYAPPRECIABLE MELT OCCURS, THE MOL RATIO OF SILICON DIOXIDE TO ALKALIMETAL CARBONATE IN SAID MIXTURE BEING WITHIN THE RANGE OF FROM 1:2 TO3:1.